A crack is a long, narrow opening resulting from the separation of two materials. In geology, two common types of cracks can be distinguished: contraction cracks and wedge cracks.

Contraction cracks are fissures relatively wide in relation to their length, which open when the soil or a rock contracts. Its formation constitutes a characteristic phenomenon of clay soils that, when dried, form a polygonal network of these shrinkage cracks. Certain layers of the subsoil preserve the traces of cracks that once opened were filled with sand, which prevented subsequent moisture from closing them again.

For their part, wedge cracks are vertical, produced mainly in the cold regions of the globe formed after the rapid freezing of the soil. They measure one or several decimeters wide (exceptionally meters), one or several meters long and up to 10 m deep. These cavities end up being filled with debris caused by the action of ice and thawing.

In glaciers, cracks are usually called by the French term crevasses, and they commonly form when the ice is subjected to tensile stresses that exceed its plasticity.